This invention relates to electrophoretic slip casting of ceramic articles.
Slip casting is commonly used to form hollow ceramic articles. In this process, an aqueous suspension (known as "slip") of a ceramic material (which phrase includes a mixture of ceramic materials), for example clay, is poured into a Plaster of Paris mould of the required shape. The mould, being porous, removes water from the slip by capillary action, and a cast of the clay or other material gradually builds up on the mould surface. When this cast is of the desired thickness, excess slip is poured out of the mould, and the cast is allowed partially to dry in the mould. During drying the cast shrinks away from the plaster mould allowing the cast to be removed, further dried and then fired, to form the finished ceramic article.
This casting process is rather slow. Also, the rheological properties of the slip are critical, with minor variations in viscosity and thixotropy resulting in casting faults.
It has been proposed to assist the casting process by means of electrophoresis. Application of a direct current potential difference between two electrodes suitably placed, one in contact with the mould and the other in the slip contained in the mould, causes a migration of the solid particles suspended in the slip to the walls of the mould to form the cast. Unfortunately, however, this potential difference simultaneously electrolyses the water of the slip, and gas evolved at the electrode in contact with the mould spoils the surface of the cast. Nonetheless, electrophoretic slip casting is still desirable as it can speed up casting by a factor of 10 or more and does not require such close control of the rheological properties of the slip. Also, by replacing Plaster of Paris moulds by stronger, conductive moulds, it allows mould life to be extended well beyond the 70 or so fillings which is typical of the life of a plaster mould used for conventional slip casting.